The present invention relates to well logging tools, and more particularly to induction logging methods and apparatus for measuring the resistivity (or its inverse, conductivity) of earth formations penetrated by a borehole.
The basic principles and techniques for induction logging of earth formations are well known. In brief, the resistivities of the various formation structures are determined by inducing eddy currents to flow in the formations in response to an AC transmitter signal, and then measuring a phase component in a receiver signal generated by the eddy currents. Usually the component of the receiver signal which is in phase with the transmitter signal is taken as indicative of the formation conductivity. With proper coil design, the output signal can be directly and linearly proportional to the electrical conductivities of the formations over formation conductivity values most commonly encountered. The output signal is then multiplied by an appropriate tool constant for recording at the surface as a function of the depth of the tool in the borehole.
One significant limitation to most current induction logging technology has been the inability to log satisfactorily through casing as commonly encountered in a cased well borehole. The measurement of formation properties behind casing has numerous important applications including monitoring of the oil/water contact, reevaluation of the formation in existing wells after production, and reservoir monitoring through observation wells. Unfortunately, attempts at induction logging through casing have thus far not been commercially satisfactory. That is, to the extent the designs worked, they were too slow or too inaccurate to be commercially acceptable.
The major problem with induction logging through casing is readily appreciated when one remembers that steel casing is about 10.sup.7 more conductive than the formations being measured. Thus the casing either "shorts out" the AC fields from the logging tool's transmitter coils (by completely blocking them if the frequency is above the casing cut-off frequency--a function of the casing thickness), or the casing effectively masks the formation signal component behind the massively greater casing signal component.
To get around this problem, one prior art technique (U.S. Pat. No. 4,748,415) utilized logging frequencies of 0.001 Hz to 20 Hz, stressing that higher frequencies would be unable to penetrate the casing. This is three to four orders of magnitude less than the 10 kHz to 40 kHz frequencies ordinarily preferred for open hole induction logging. U.S. Pat. No. 4,499,422, for example, specifies this frequency range to optimize the trade-offs between skin effect, tool sensitivity, and resolution for open hole induction logging.
Because of the lack of acceptable through casing induction technology, the major current methods for measuring formation properties behind casing rely essentially on nuclear technology. These techniques include, for example, neutron thermal decay logs and gamma spectroscopy logs. Notably, there has been considerable interest in being able to perform induction logging in cased holes. Certainly, in open holes the commercial preference has been for resistivity type measurements rather than thermal decay logs.
A need therefore remains for methods and apparatus for high resolution induction logging through casing in a borehole, and in particular, for such methods and apparatus which can be performed economically, accurately, and rapidly enough to be commercially viable.